The invention relates to a disk prosthesis for cervical vertebrae, intended to replace the fiber and cartilage disk that interconnects cervical vertebrae in the spinal column.
It is known that an intervertebral disk can be subject to damage, such as compression, deformation, displacement, or wear, and more generally degeneration associated with the mechanical stresses that are applied thereto and that lead to anatomical and functional destruction of the disk and of the vertebral segment. This damage to the disk alters its mechanical behavior and leads to a reduction in the height of the intersomatic gap, which leads to all of the functions of the joint being disturbed. This gives rise to instability which induces, in particular, an osteoarthritic reaction which is painful and gives rise to osteophytic processes.
Proposals have therefore been made to replace the defective disk with an artificial disk, and various types of embodiment have been envisaged. Thus, a disk prosthesis is known, e.g. from patent FR 2 718 635, for cervical vertebrae where the prosthesis comprises first and second plates for fixing to adjacent cervical vertebrae. That prosthesis also has a ball joint interposed between the two plates that are mounted in a superposed position. The joint comprises a spherical cap made of a synthetic material, such as polyethylene, mounted on one of the plates and designed to co-operate with a spherical cup provided in the other plate and made of a metal, such as titanium, for example.
Although such a cervical prosthesis makes it possible to restore an appropriate height to the intersomatic gap, the ball joint of that prosthesis is subject to high levels of friction, making it sensitive to wear, and as a consequence of the wear that prosthesis does not give full satisfaction firstly because of its relative instability, particularly in bending movements.
A disk prosthesis for lumbar vertebrae is also known, in particular from patent U.S. Pat. No. 5,562,738, which comprises first and second plates for fixing to adjacent vertebrae, and made of a metal, such as titanium. Between the plates, a ball joint is interposed that comprises a first insert mounted on one of the plates and constituted by a spherical cap co-operating with a spherical cup of a second insert mounted on the other plate. The inserts are made of a biocompatible ceramic material having improved tribological characteristics, in particular concerning resistance to wear.
Nevertheless, such a disk prosthesis for lumbar vertebrae is unsuitable for replacing the disk of cervical vertebrae insofar as such a prosthesis does not enable cervical vertebrae to recover their natural mobility. It also turns out that the ball joint presents a shape that is quite difficult to get right and that is sensitive to breaking or cracking phenomena that reduce the lifetime of the prosthesis.
The object of the present invention is thus to remedy the drawbacks of the state of the art by proposing a disk prosthesis for cervical vertebrae that is designed to present relative long lifetime by being practically insensitive to phenomena of wear and breaking, and while being suitable for providing physiological mobility between the two cervical vertebrae to which it is fitted.
To achieve this object, the prosthesis of the invention is of the type comprising:
first and second plates designed to be fixed to adjacent cervical vertebrae; and
a ball joint interposed between the two plates mounted in a superposed position, the joint being constituted by a spherical cap co-operating with a spherical cup.
According to the invention:
the spherical cap is provided on a first insert, while the spherical cup is provided on a second insert;
each insert is made of a ceramic material and possesses a base of circular right cross-section;
one of the inserts is mounted on the first plate while the other insert is mounted on the second plate in such a manner that the center of rotation of the joint lies substantially centered relative to the edges of the plates so as to be centered in the sagittal plane and in the frontal plane of the vertebrae;
the spherical cup possesses a contact area that is not less than the contact area of the spherical cap and is connected via an annular molding to the base of the insert; and
the plate provided with the insert having the spherical cap has an annular setback to leave clearance for the annular molding of the spherical cup during movements of the plates.